This invention relates generally to devices for blowing preheated air into a shaft furnace. More particularly, this invention relates to a device for blowing preheated air into a shaft furnace consisting of several separate tubular elements comprising an outer steel shielding and an inner lining made of cast refractory concrete, connected in pairs by a ball joint and a compensator.
Preheated air blowing devices, more generally known under the name "tuyere stock", are subject to problems of mobility and leaktightness. In fact, as a consequence of the high temperature of the preheated air (temperature on the order of 1200.degree. C. or more) and the high temperature prevailing inside the furnace, the wall of the furnace, as well as the supply pipe and the tuyere stock are exposed to thermal expansions and deformations causing significant relative displacements between the supply pipe and the wall of the furnace. It is therefore necessary for the tuyere stock to be capable of compensating for these relative displacements while at the same time, avoiding leaks of gas or preheated air.
In order to satisfy these requirements, U.S. Pat. No. 3,766,868 provides a tuyere stock of the type described hereinabove. This tuyere stock has subsequently been improved by the design of universal ball joints of the type described in document DE-C2-2218331. The joints of this tuyere stock make it possible to compensate for all the relative movements between the supply pipe and the wall of the furnace. The leaktightness at the joints is ensured by bellows compensators, while the mechanical stability is ensured, for example by associated universal linkages, at the two universal joints, at the two opposite ends of the central tubular element.
By far, the most greatly stressed and least resilient location is always situated at the joints. In fact, the mobility of the pivot with respect to its dish often leads to rubbing of refractory concrete on refractory concrete. As a result, the refractory concrete at the tip of the pivot can be damaged and splinters can possibly be detached. Thus, in view of the large variations in temperature, micro-cracks can form at certain places, which cause detrimental circulation and turbulence.
U.S. Pat. No. 5,209,657 proposes extension of the metal shielding forming the sheath of the pivot as far as the diametrical base of the convex pivot. This invention greatly improves the mechanical stability of the convex pivots and allows better sliding of the convex pivot in the concave dish of the adjacent segment.
However, in the long term, the refractory steel forming the tip of the pivot oxidizes, and the entire sheath of the pivot must be replaced.
It will be appreciated that there remains an urgent need to develop and provide a new and improved system of joints that is much more resistant both to the thermal and mechanical stresses to which such joints are subject to in the field.